REVIEWS 207 



preglacial features are for the most part concealed. The drainage 

 systems are comparatively young, although the streams have the advan- 

 tage of working on a surface of higher altitude and greater diversity of 

 relief than that of the older drift of central and southern Illinois 

 ■Streams follow the axes of drift basins included between successive 

 morainic ridges, their longer tributaries being carried on the longer 

 slopes of these basins which lie toward the west and south. A pre- 

 glacial divide is traced from below Elgin and Lemont to the Indiana 

 state line, and it is highly probable that the headwaters of the Fox, 

 Des Plaines and Kankakee rivers were in preglacial times tributary to 

 the Lake Michigan basin. In northwestern Illinois instances are noted 

 of displacement of the rivers and their beheading by drift deposits. 

 The data supplied by artesian well records suggests several important 

 conclusions as to the deeper strata. The altitudes of the St. Peter 

 sandstone and the base of the Coal Measures are worked out in detail 

 •over much of the state. Three thousand feet is considered a liberal 

 estimate for the thickness of the Palfeozoic formations of northern Illi- 

 nois. A maximum of 6000 feet is set for the thickness of the Palceozoic 

 in southern Illinois, of which from 1200 to 1500 feet is allotted to the 

 Coal Measures. The terms Potsdam and Lower Magnesian are retained, 

 and a thickness is assigned to the latter at Rock Island of about 

 800 feet. In this instance it seems probable to the reviewer that this 

 measure includes the Jordan and Saint Lawrence as well as the Oneota 

 or Lower Magnesian. Remarkable variations in well records are noted, 

 and, like all workers with such data, the author has no doubt felt the 

 embarrassment of riches when more than one well record is extant in 

 any district. At Chicago, for example, the recorded thickness of the 

 St. Peter sandstone ranges from 89 to 420 feet. Surely in the latter 

 measurement the driller, or the authority for the record, has either 

 reckoned in arenaceous beds of the Oneota and the New Richmond, or 

 has been misled by St. Peter sand in drillings far below the lower limit 

 of the formation. 



The Potsdam, the St. Peter, the Galena, the Lower Magnesian, and 

 the Niagara, are the chief artesian water-bearing strata. 



The author does not find it easy to separate flowing from non-flow- 

 ing wells in which water rises under hydrostatic pressure, and desig- 

 nates both classes as artesian. He instances wells in Chicago which 

 pass from one class to the other each week, flowing only for a brief 

 period after the Sundav intermission from pumping of neighboring wells. 



